1. Field of the Invention
This invention relates to digital information or data transmission along an optical bidirectional transmission medium.
More particularly, the invention concerns a digital information transmitting system that essentially comprises several stations, each including an emitter and a receiver, and information processing means for managing the information emissions and receptions between the stations. The processing means and the stations are interconnected to the optical transmission medium such that an emitter of a given station may communicate with the receivers of one or more given stations. The optical transmission medium usually, called an optical bus includes one or more optical fibers in a garland or star array connecting the emitters and receivers via the processing means. The information transmitted in-line usually takes the form of a series of luminous pulses or bits resulting from a known coding operation, e.g. pulse code modulation (PCM).
A message is formed of a series of bits directed from an emitter of a given station toward one or more receivers of other stations. Besides useful data, this message can comprise service, synchronization and address information along with any other information required for the message transmission.
2. Description of the Prior Art
Optical bus transmission systems are already known. French patent application 2,473,823 describes an optical bus transmission system in which each emitter communicates with the other emitters through an optical transmission medium called a bus. In response to each data transmission, a processing means authorizes a station to emit and authorizes at least one other station to receive; to this end the processing means sends orders along the bus, in accordance with a given exchange procedure. The optical fiber garland-arrayed systems cannot, as a result of the optical link analysis, accept a high number of shunt and insertion branches on a single optical line. Those lines that carry repeater-regenerators introduce phase shifts and additional noise, giving rise to a reduction in the useful pass band of the line and a limit on the maximum distance between remote subscriber stations.
The systems making use of star-array passive couplers introduce considerable attenuation in the optical signal. Such systems on special components that are difficult to double up and which determine the optical transmission characteristics.
Furthermore, should a subscriber station become defective, the optical bus can be disturbed and adversely affect the transmission of other messages. Seeking out the failure or checking the circuits induces disturbance of the same nature. Such systems provide periodic processing of the bus lines in order to attribute an emission time to each station. Lastly, more recent systems include a programming mode covering message transfer along the bus in order to provide bus access possibilities at times chosen by the subscriber. However, the precise moment when the bus becomes available depends on the traffic at the particular instant along the multiple subscriber lines connected to the bus and on the system configuration.